Programmed death receptor 1 (PD-1) is primarily expressed on lymphocytes and has two ligands, PD-L1 and PD-L2. PD-1 is a 55 kDa protein encoded by a gene Pdcd1 and was shown to down-regulate antigen receptor signaling driven by its ligand's engagement (Freeman et al. (2000) J Exp Med 192:1027-34; Latchman, et. al. (2001) Nat Immunol 2:261-8; Carter et al. (2002) Eur J Immunol 32:634-43). PD-1 belongs to the immunoglobulin superfamily which includes members such as CD28, CTLA-4, ICOS and BTLA. PD-1 is type I transmembrane glycoprotein containing an Ig variable-type (V-type) domain for ligand binding and a cytoplasmic tail for the binding of signaling molecules. PD-1 contains two cytoplasmic tyrosine-based signaling motifs, an immunoreceptor tyrosine-based inhibition motif (ITIM) and an immunoreceptor tyrosine-based switch motif (ITSM). Following T cell stimulation, PD-1 recruits the tyrosine phosphatase SHP-2 to the ITSM motif within its cytoplasmic tail, leading to the dephosphorylation of effector molecules such as CD3 Zeta, PKC theta and ZAP70 that are involved in the CD3 T cell signaling cascade. In contrast, PD-1's ligands (PD-L1 and PD-L2) have two short cytoplasmic regions with no known functions. The ligands have an extracellular region containing IgV- and IgC-like domains and are constitutively expressed or can be induced in a variety of cell types, including non-hematopoietic tissues as well as various tumor types. PD-L1 is not only expressed on B, T, myeloid and dendritic cells (DCs), but also on peripheral cells, like microvascular endothelial cells and non-lymphoid organs like heart, lung etc. In contrast, PD-L2 is only found on macrophages and DCs. The expression pattern of PD-1 ligands is suggestive of a role for PD-1 in maintaining peripheral tolerance and may serve regulate self-reactive T- and B-cell responses in the periphery. To date, numerous studies have shown that interaction of PD-1 with its ligands leads to the inhibition of lymphocyte proliferation in vitro and in vivo. Disruption of the PD-1/PDL1 interaction has been shown to increase T cell proliferation and promote cytokine production.
Thus, there is an important role for the PD-1/PD-L1 pathway in controlling immune responses. Dysfunction of PD-1/PD-L1 signaling appears to be correlated with initiation and development of diseases such as cancer and viral infection. Analysis of knockout animals has led to the understanding that PD-1 functions mainly in inducing and regulating peripheral tolerance. Thus, therapeutic blockade of the PD-1 pathway would be helpful in overcoming immune tolerance and in the treatment of cancer or infection as well as in boosting immunity during vaccination (either prophylactic or therapeutic). There is a need in the art for improved methods for blocking the PD-1 pathway.